Ischemia is the leading cause of illness and disability in the world. Ischemia is a deficiency of oxygen in a part of the body causing metabolic changes, usually temporary, which can be due to a constriction or an obstruction in the blood vessel supplying that part. The two most common forms of ischemia are cardiovascular and cerebrovascular. Cardiovascular ischemia, in which the body's capacity to provide oxygen to the heart is diminished, is the leading cause of illness and death in the United States. Cerebral ischemia is a precursor to cerebrovascular accident (stroke) which is the third leading cause of death in the United States.
The continuum of ischemic disease includes five conditions: (1) elevated blood levels of cholesterol and other blood lipids; (2) subsequent narrowing of the arteries; (3) reduced blood flow to a body organ (as a result of arterial narrowing); (4) cellular damage to an organ caused by a lack of oxygen; (5) death of organ tissue caused by sustained oxygen deprivation. Stages three through five are collectively referred to as “ischemic disease,” while stages one and two are considered its precursors.
Together, cardiovascular and cerebrovascular disease accounted for 954,720 deaths in the U.S. in 1994. Furthermore, more than 20% of the population has some form of cardiovascular disease. In 1998, as many as 1.5 million Americans will have a new or recurrent heart attack, and about 33% of them will die. Additionally, as many as 3 to 4 million Americans suffer from what is referred to as “silent ischemia.” This is a condition where no clinical symptoms of ischemic heart disease are present.
There is currently a pressing need for the development and utilization of blood tests able to detect injury to the heart muscle and coronary arteries. Successful treatment of cardiac events depends largely on detecting and reacting to the presence of cardiac ischemia in time to minimize damage. Cardiac enzymes, specifically the creatine kinase isoenzyme (CK-MB), and cardiac markers, specifically the Troponin I and T biochemical markers, are utilized for diagnosing heart muscle injury. However, these enzymes and markers are incapable of detecting the existence of an ischemic state in a patient prior to myocardial infarction and resulting cell necrosis (death of cell). Additionally, these enzymes and markers do not show a measurable increase until several hours after an ischemic event. For instance, CK-MB, the earlier evident of the two, does not shows a measurable increase above normal in a person's blood test until about four to six hours after the beginning of a heart attack and does not reach peak blood level until about 18 hours after such an event. Thus, the primary shortcoming of using cardiac markers for diagnosis of ischemic states is that these markers are only detectable after heart tissue has been irreversibly damaged.
There currently are no tests available which allow diagnosis of the existence of ischemia in patients prior to tissue necrosis. A pressing requirement for emergency medicine physicians who treat chest pain and stroke symptoms is for a diagnostic test that would enable them to definitively “rule out” myocardial infarction, stroke, and other emergent forms of ischemia. A need exists for a method for immediate and rapid distinction between ischemic and non-ischemic events, particularly in patients undergoing acute cardiac-type symptoms. The present invention provides such a means.
A broader array of diagnostic tests are available for diagnosis of ischemia in patients with non-acute symptoms. The EKG exercise stress test is commonly used as an initial screen for cardiac ischemia, but is limited by its accuracy rates of only 25-50%. Coronary angiography, an invasive procedure that detects narrowing in the arteries with 90-95% accuracy, is also utilized. Another commonly used diagnostic test is the thallium exercise stress test, which requires injection of radioactive dye and serial tests conducted four hours apart. The present invention, however, has the advantage over the known methods of diagnosis in that it provides equivalent or better accuracy at far lower costs and decreased risk and inconvenience to the patient. The present invention provides specificity and sensitivity levels of 75-95%, which are far more accurate than the EKG exercise stress test and comparable in accuracy to current diagnostic standards. Furthermore, the present invention presents a significant time advantage and is cheaper than competing methods of diagnosis by a factor of at least 15 to 1.
It is known that immediately following an ischemic event, proteins (enzymes) are released into the blood. Well known proteins released after an ischemic heart event include creatine kinase (CK), serum glutamic oxalacetic transaminase (SGOT) and lactic dehydrogenase (LDH). One well known method of evaluating the occurrence of past ischemic heart events is the detection of these proteins in a patient's blood. U.S. Pat. No. 4,492,753 relates to a similar method of assessing the risk of future ischemic heart events. However, injured heart tissue releases proteins to the bloodstream after both ischemic and non-ischemic events. For instance, patients undergoing non-cardiac surgery may experience perioperative ischemia. Electro-cardiograms of these patients show ST-segment shifts with an ischemic cause which are highly correlated with the incidence of postoperative adverse cardiac events. However, ST-segment shifts also occur in the absence of ischemia; therefore, electrocardiogram testing does not distinguish ischemic from non-ischemic events. The present invention provides a means for distinguishing perioperative ischemia from ischemia caused by, among other things, myocardial infarctions and progressive coronary artery disease.